A couple of comments on refractometers.
(1) I brew with my tap water, so that's what I used to calibrate it. My thinking is that this will "zero out" any gravity contribution from the minerals or whatever that are in my tap water.
(2) refractometer readings are only accurate "as-is" for OG readings. Once fermentation has begun, the alcohol present in the beer will throw off the reading of the refractometer. But, if you have the pre-fermentation refractometer reading, there is some math you can do with to convert a post-fermentation reading to gravity. Here's a link to an on-line calculator that will do this for you: http://www.onebeer.net/refractometer.shtml